1. Technical Field
The present invention relates in general to a magnetic media storage system and in particular, to a magnetic tape storage system utilizing a servo system to position a read/write head over parallel data tracks. Still more particularly, the present invention relates to a method and system for generating a position error signal for use in parallel track tape storage systems utilizing servo systems to dynamically position a read/write head over parallel data tracks.
2. Description of the Related Art
In high track density magnetic storage systems, manufacturing tolerances, tape interchange, and thermal and hygroscopic variations all cause track misalignment of a multitrack tape format. Track densities on magnetic tape are limited by the maximum possible track misalignment between the magnetic tracks on the tape and the transducer tracks in the read/write head. Reducing the misalignment of the read/write head by using a servo system to guide the read/write head relative to the magnetic tracks allows the track width and guardbands to be decreased, thereby increasing recording densities and resulting in better product performance.
Misalignment may be partially overcome utilizing an indexing mechanism that performs initial alignment, but does nothing to correct dynamic misalignments. A solution for dynamically correcting read/write head position involves continuously sensing head-to-track position as required for closed loop track following.
The current and prior art for obtaining servo information from magnetic recording media fall into three categories, which may be used singly or in concert to correct misalignment between the data tracks and the transducer elements:
(a) Isolated bursts of positioning information are strategically placed in the format of the storage medium to allow correction of manufacturing tolerances and machine-to-machine variations (e.g., as taught in IBM Technical Disclosure Bulletin, Vol. 31, No. 11, 04-89, p.219, by Cannon).
(b) Periodic samples of positioning information may be regularly interspersed with the recorded data (e.g., as cited in Research Disclosure n.328, 08-91), with the positioning information consisting of any of several well known recorded patterns, such as single frequency bursts alternately displaced to either side of the desired position, or multiple frequencies recorded on adjacent tracks. Within the limitations imposed by the sampling process, periodic and quasi-periodic alignment variations may be sensed and compensating corrections made.
(c) Continuous tracks of positioning information may be provided on the recording medium. This information may be physically inscribed, included as a magnetic sub-layer (both described in IBM Technical Disclosure Bulletin, Vol. 23, No. 2, 07-80, pp.787-789, by Gruss & Tietze), magnetically recorded in a formatting process a priori use for data storage (e.g., IBM Technical Disclosure Bulletin, Vol. 19, No. 3, 08-76, pp.810-813, by Lewkowiez & Stephens), or recorded concurrently with the recording of data (e.g., IBM Technical Disclosure Bulletin, Vol. 25, No. 2, 07/82, pp.778-779, by T. A. Schwarz). Continuity of the position information permits detection and correction of transient (non-periodic, non-repeatable) misregistration conditions.
The positioning information is recorded in physically (or logically) adjacent regions on the medium, and typically consists of magnetic patterns differing in frequency, phase, encoded data, physical or time position. A Position Error Signal is derived by detecting changes in amplitude or phase from nominal; thus, noise occurring as amplitude perturbations or as timing jitter corrupts the detected positioning information. Satisfactory performance of the servo systems in each category depends on complicated and expensive servo writing systems.
It would be desirable to have an improved method and system, in a high track density tape storage system utilizing a continuous track following servo head positioner, for generating a position error signal to correct head/track misregistration requiring no precision servo writer while minimizing noise induced errors.